The Epigenetics Revolution (42 page)
Read The Epigenetics Revolution Online
Authors: Nessa Carey
Tags: #Science/Life Sciences/Genetics and Genomics
However, a new queen and her clonal worker sisters are clearly incredibly different from each other, both in physical form and in activities. The queen can be twice the size of a worker bee. After the so-called nuptial flight, when she first leaves a colony and mates, the queen almost never leaves the hive again. She stays in the darkness of the interior, laying up to 2,000 eggs a day in the summer months. She has no sting barbs, no wax glands and no pollen baskets (not much point having a shopping bag if you never leave the house). Worker bees have a lifespan that can usually be measured in weeks, whereas queens live for years
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Conversely, workers can do many things that the queens can’t. Chief amongst these is collecting food, and then telling the rest of the colony its location. This information is communicated using the famous ‘waggle dance’. The queen lives in darkened luxury, but she never gets to boogie.
So, a honeybee colony contains thousands of individuals who are genetically identical, but a few of them are really different physically and behaviourally. This difference in outcome is all down to how the bee larvae are fed. The pattern of early feeding completely determines whether a larva will develop into a worker or into a queen.
For honeybees the DNA script is constant but the outcome is variable. The outcome is controlled by an early event (feeding pattern) which sets a phenotype that is maintained throughout the rest of life. This is a scenario that just shrieks epigenetics at us, and in the last few years scientists have started to unravel the molecular events that underpin this process.
The critical roll of the dice for honeybees happens after the third day of life, as a fairly immobile grub or larva. Until day three, all honeybee larvae are given the same food. This is a substance called royal jelly, which is produced by a specialised group of workers. These young workers are known as nurse bees and they secrete royal jelly from glands in their heads. Royal jelly is a highly nutritious food source. It is a concentrated mix of a lot of different components, including key amino acids, unusual fats, specific proteins, vitamins and other nutrients that haven’t been well-characterised yet.
Once the larvae are three days old, the nurse bees stop feeding royal jelly to most of them. Instead, most larvae are switched onto a diet of pollen and nectar. These are the larvae which will grow up to be worker bees.
But for reasons that nobody really understands, the nurse bees continue feeding royal jelly to a select few larvae. We don’t know how these larvae are chosen or why. Genetically they are identical to the ones that are switched onto the less sophisticated diet. But this small group of larvae that continue to be nourished with royal jelly grow up to be queens, and they’re fed this same substance throughout their lives. The royal jelly is essential for the production of mature ovaries in the queens. Worker females never develop proper ovaries, which is one of the reasons they are infertile. Royal jelly also prevents the queen from developing the organs that she won’t ever need, like those pollen baskets.
We understand some of the mechanisms behind this process. Bee larvae contain an organ that has some of the same functions as our liver. When a larva receives royal jelly continuously, this organ processes the complex food source and activates the insulin pathway. This is very similar to the hormonal pathway in mammals that controls the levels of sugar in the bloodstream. In honeybees activation of this pathway increases production of another hormone, called Juvenile Hormone. Juvenile Hormone in turn activates other pathways. Some of these stimulate growth and development of tissues like the maturing ovaries. Others shut down production of the organs that the queen doesn’t need
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Mimicking royalty
Because honeybee maturation has so many hallmarks of an epigenetic phenomenon, researchers speculated that there would also be an involvement of the epigenetic machinery. The first indications that this is indeed the case came in 2006. This was the year when researchers sequenced the genome of this species, to identify the fundamental genetic blueprint
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. Their research showed that the honeybee genome contained genes that looked very similar to the DNA methyltransferase genes of higher organisms such as vertebrates. The honeybee genome was also shown to contain a lot of CpG motifs. This is the two-nucleotide sequence that is usually the target for DNA methyltransferases.
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. Their research showed that the honeybee genome contained genes that looked very similar to the DNA methyltransferase genes of higher organisms such as vertebrates. The honeybee genome was also shown to contain a lot of CpG motifs. This is the two-nucleotide sequence that is usually the target for DNA methyltransferases.
In the same year, a group led by Gene Robinson in Illinois showed that the predicted DNA methyltransferase proteins encoded in the honeybee genome were active. The proteins were able to add methyl groups to the cytosine residue in a CpG motif in DNA
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. Honeybees also expressed proteins that were able to bind to methylated DNA. Together, these data showed that honeybee cells could both ‘write’ and ‘read’ an epigenetic code.
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. Honeybees also expressed proteins that were able to bind to methylated DNA. Together, these data showed that honeybee cells could both ‘write’ and ‘read’ an epigenetic code.
Until these data were published, nobody had really wanted to take a guess as to whether or not honeybees would possess a DNA methylation system. This was because the most widely used experimental system in insects, the fruit fly
Drosophila melanogaster
, whom we met earlier in this book, doesn’t methylate its DNA.
Drosophila melanogaster
, whom we met earlier in this book, doesn’t methylate its DNA.
It’s interesting to discover that honeybees have an intact DNA methylation system. But this doesn’t prove that DNA methylation is involved in the responses to royal jelly, or the persistent effects of this foodstuff on the physical form and behaviour of mature bees. This issue was addressed by some elegant work from the laboratory of Dr Ryszard Maleszka at the Australian National University in Canberra.
Dr Maleszka and his colleagues knocked down the expression of one of the DNA methyltransferases in honeybee larvae, by switching off the
Dnmt3
gene. Dnmt3 is responsible for adding methyl groups to regions of DNA that haven’t been methylated before. The results of this experiment are shown in
Figure 14.1
.
Dnmt3
gene. Dnmt3 is responsible for adding methyl groups to regions of DNA that haven’t been methylated before. The results of this experiment are shown in
Figure 14.1
.
Figure 14.1
When royal jelly is fed to honeybee larvae for extended periods, the larvae develop into queens. The same effect is seen in the absence of prolonged feeding with royal jelly if the expression of the
Dnmt3
gene is decreased experimentally in the larvae. Dnmt3 protein adds methyl groups to DNA.
When royal jelly is fed to honeybee larvae for extended periods, the larvae develop into queens. The same effect is seen in the absence of prolonged feeding with royal jelly if the expression of the
Dnmt3
gene is decreased experimentally in the larvae. Dnmt3 protein adds methyl groups to DNA.
When the scientists decreased the expression of
Dnmt3
in the honeybee larvae, the results were the same as if they had fed them royal jelly. Most of the larvae matured as queens, rather than as workers. Because knocking down
Dnmt3
had the same effects as feeding royal jelly, this suggested that one of the major effects of royal jelly is connected with alterations of the DNA methylation patterns on important genes
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.
Dnmt3
in the honeybee larvae, the results were the same as if they had fed them royal jelly. Most of the larvae matured as queens, rather than as workers. Because knocking down
Dnmt3
had the same effects as feeding royal jelly, this suggested that one of the major effects of royal jelly is connected with alterations of the DNA methylation patterns on important genes
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.
To back up this hypothesis, the researchers also examined the actual DNA methylation and gene expression patterns in the different experimental groups of bees. They showed that the brains of queens and worker bees have a different DNA methylation pattern. The DNA methylation patterns in the bees where
Dnmt3
had been knocked down were like those of the normal royal jelly-induced queens. This is what we would expect given the similar phenotypes in the two groups. The gene expression patterns in the normal queens and the
Dnmt3
-knockdown queens were also very similar. The authors concluded that the nutritional effects of continual feeding on royal jelly occurred via DNA methylation.
Dnmt3
had been knocked down were like those of the normal royal jelly-induced queens. This is what we would expect given the similar phenotypes in the two groups. The gene expression patterns in the normal queens and the
Dnmt3
-knockdown queens were also very similar. The authors concluded that the nutritional effects of continual feeding on royal jelly occurred via DNA methylation.
There are still a lot of gaps in our understanding of how nutrition in the honeybee larva results in altered patterns of DNA methylation. One hypothesis, based on the experiments above, is that royal jelly inhibits the DNA methyltransferase enzyme. But so far nobody has been able to demonstrate this experimentally. It’s therefore possible that the effect of royal jelly on DNA methylation is indirect.
What we do know is that royal jelly influences hormonal signalling in honeybees, and that this changes gene expression patterns. Changes in the levels of expression of a gene often influence the epigenetic modifications at that gene. The more highly a gene is switched on, the more its histones become modified in ways which promote gene expression. Something similar may happen in honeybees.
We also know that the DNA methylation systems and histone modification systems often work together. This has created interest in the role of histone-modifying enzymes in the control of honeybee development and activity. When the honeybee genome was sequenced, four histone deacetylase enzymes were identified. This was intriguing because it has been known for some time that royal jelly contains a compound called phenyl butyrate
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. This very small molecule can inhibit histone deacetylases but it does so rather weakly. In 2011, a group led by Dr Mark Bedford from the MD Anderson Cancer Center in Houston published an intriguing study on another component of royal jelly. One of the other senior authors on this paper was Professor Jean-Pierre Issa, who has been so influential in promoting use of epigenetic drugs for the treatment of cancer.
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. This very small molecule can inhibit histone deacetylases but it does so rather weakly. In 2011, a group led by Dr Mark Bedford from the MD Anderson Cancer Center in Houston published an intriguing study on another component of royal jelly. One of the other senior authors on this paper was Professor Jean-Pierre Issa, who has been so influential in promoting use of epigenetic drugs for the treatment of cancer.
The researchers analysed a compound found in royal jelly called (E)-10-hydroxy-2-decenoic acid, or 10HDA for short. The structure of this compound is shown in
Figure 14.2
, along with SAHA, the histone deacetylase inhibitor we saw in
Chapter 11
that is licensed for the treatment of cancer.
Figure 14.2
, along with SAHA, the histone deacetylase inhibitor we saw in
Chapter 11
that is licensed for the treatment of cancer.
The two structures aren’t identical by any means, but they do share some similarities. Each has a long chain of carbon atoms (the bit that looks like a crocodile’s back in profile), and the right hand side of each compound also looks fairly similar. Mark Bedford and his colleagues hypothesised that 10HDA might be an inhibitor of histone deacetylases. They performed a number of test tube and cell experiments and showed that this was indeed the case. This means that we now know that one of the major compounds found in royal jelly inhibits a key class of epigenetic enzymes
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Figure 14.2
The chemical structure of the histone deacetylase inhibitor SAHA and 10HDA, a compound found in royal jelly. C: carbon; H: hydrogen; N: nitrogen; O: oxygen. For simplicity, some carbon atoms have not been explicitly shown, but are present where there is a junction of two lines.
The chemical structure of the histone deacetylase inhibitor SAHA and 10HDA, a compound found in royal jelly. C: carbon; H: hydrogen; N: nitrogen; O: oxygen. For simplicity, some carbon atoms have not been explicitly shown, but are present where there is a junction of two lines.
The forgetful bee and the flexible toolkit
Epigenetics influences more than whether bees develop into workers or queens. Ryszard Maleszka has also shown that DNA methylation is involved in how honeybees process memories. When honeybees find a good source of pollen or nectar, they fly back to the hive and tell the other members of the colony where to head to find this great food supply. This tells us something really important about honeybees; they can remember information. They must be able to, or they wouldn’t be able to tell the other bees where to go for food. Of course, it’s equally important that the bees can forget information and replace it with new data. There’s no point sending your co-workers to the great patch of thistles that were in flower last week, but that have now been eaten by a grazing donkey. The bee needs to forget last week’s thistles and remember the location of this week’s lavender.
It’s actually possible to train honeybees to respond to certain stimuli associated with food. Dr Maleszka and his colleagues showed that when the bees undergo this training, the levels of Dnmt3 protein go up in the parts of the honeybee brains which are important in learning. If the bees are treated with a drug that inhibits the Dnmt3 protein, the compound alters the way the bees retain memories, and also the speed with which memories are lost
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Although we know that DNA methylation is important in honeybee memory, we don’t know exactly how this works. This is because it’s not clear yet which genes become methylated when honeybees learn and acquire new memories.
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